1. Field of the Present Invention
The present invention relates to a signal-off detection circuit and an optical receiving device using the detection circuit and, in particular, to improvement of a signal-off detection circuit used in optical receiving devices.
2. Description of the Prior Art
When data signals to an optical receiving device are disconnected, that is, in the case of a break in the optical fiber and in the case of a failure of OFA (Optical Fiber Amplifier) in a WDM (Wave Length Division Multiplex) scheme, a function detecting a transmission line failure and issuing an alarm (LOS: Loss Of Signal) is required for isolating a failure to a transmission line failure and a device failure. Thus, this function is prescribed as an essential function by ITU (International Telecommunication Union).
When this alarm is issued, the system checks the operating state of its opposite device and performs operations such as switching the transmission line to a standby system (such an operation control is performed by a network management system of each device). After that, when all abnormal conditions have been resolved, the network management system checks communication to and from the opposite device and switches back from the standby system to the current use system. In this way, the alarm signal basically acts as a trigger for carrying out automatic switching between the system and the opposite device, and thus reliable operation for issuing alarms is required.
Further, although an issuing time of a signal disconnection alarm is not particularly defined by ITU, the Bellcore standard requires alarm-issuing at 2.3 s to 100 s of a data signal-off. In this case, in consideration of the switching of a cross-connecting apparatus or the like and continuation of the same symbol, it is required that a signal disconnection alarm should not be issued within 2.3 s of a data signal-off.
In an example of a conventional signal-off detection circuit for issuing such a signal disconnection alarm, there is a configuration shown in FIG. 12A. Referring to FIG. 12A and FIG. 12B showing operating waveforms of the circuit of FIG. 12A, the configuration is as follows. An input data signal is converted from an optical signal to an electrical signal, which is amplified to a fixed amplitude by an amplifier 20. Then, waveforms of the input data signal in the lower side than a direct current offset voltage Vdc are folded back to the upper side in a full-wave rectifying circuit 30, thus providing a peak value of the data signal, and the peak value is detected in a peak value detecting circuit 31. If the detected peak value is equal to or less than a reference voltage of a comparator 32, a signal disconnection alarm is issued.
By the way, the direct current offset voltage Vdc added to the input of the amplifier 20 is generated by a direct current feedback circuit 21, wherein the offset voltage Vdc is assumed to be the direct current component of the output of the amplifier 20.
Also, another example of the signal-off detection circuit is disclosed in Japanese Patent Laid-Open No. 5-91148, and its configuration is shown in FIG. 13 and examples of its operation wave forms are shown in FIG. 14. In FIG. 13, an interval generation circuit 1 generates a monitoring interval signal S2 from an input signal S1 and outputs it to a flip flop (D-type F F) 2A, and at the same time, outputs a signal S5 having the same period as the interval signal S2 to a flip flop (D-type F F) 3A.
In the flip flop 2A, based on the monitoring interval signal S2 input thereto, a signal pulse to-be-monitored S3 within the time between the monitoring interval signals is detected, and a detection signal S4 of the detection result is output to a judgement flip flop 3A. The judgement flip flop 3A is configured to examine the state of the detection signal S4 for each of the monitoring interval times and output a signal-off detection information S6.
In the signal-off detection circuit having such a configuration, the interval generation circuit 1 divides the input signal S1 to generate a monitoring interval signal S2 such as shown in a timing chart of FIG. 14, and then outputs the signal S2 to the flip flop 2A and, at the same time, outputs a judgement signal S5 having the same period as the interval signal S2 to the flip flop 3A.
In the flip flop 2A, due to the input of the monitoring interval signal S2, when the interval signal S2 is “L”, initialization is performed in a unit of the interval. “H” is always input to the data terminal of the flip flop 2A. When at least one pulse of the signal to-be-monitored S3 is input within the interval time after the initialization, the flip flop 2A outputs the detection signal S4 of “H” showing normality of signals. On the other hand, when no signal to-be-monitored S3 is input within the interval time after the initialization, the flip flop 2A outputs the detection signal S4 of “L” showing a signal-off.
In the flip flop 3A, the detection signal S4 just before the initialization performed by the monitoring interval signal S2 is judged based on the judgement signal S5 input. Then, if normal, the signal-off detection information S6 of “H” is output, and if a signal-off is judged, the signal-off detection information S6 of “L” is output.
However, in the conventional configuration shown in FIG. 12A, the output voltage of the peak value detection circuit 31 after the signal-off converges on the offset voltage Vdc according to the time constant of the direct current feedback circuit 21, as shown in FIG. 12B. Therefore, the time of alarm-issuing depends on the time constant of the direct current feedback circuit 21. Generally, for the sake of the stability of control system, the time constant is required to be large, which is resulting in much time required from a signal-off to a signal disconnection alarm-issuing. For this reason, for a system in which the issuing time of a signal disconnection alarm is defined, design becomes difficult.
Also, in the conventional circuit configuration shown in FIG. 13, if only one pulse of the input signal S3 is input within one monitoring interval time, the flip flop 2A detects the one pulse and outputs the detection signal S4 of “H”. Therefore, for a system requiring that when input signals present within this interval time are equal to or more than a certain defined number (a number more than 1), then data input is judged as being present, and when less than the number, data input is judged as being absent, the circuit of FIG. 13 can not be used.
That is, the conventional example of FIG. 13 uses a scheme in which if at least one input signal is present within the monitoring interval time, input signals is judged as being present and, otherwise, input signals is judged as being absent. Therefore, it is impossible to use the convention example of FIG. 13 for a signal-off detection circuit in an optical receiving device for receiving an optical signal. This reason is because an optical receiving device requires the fact that when the number of data signals present within a certain period is equal to or more than a defined value, data input is judged as being present, and if less than the value, data input is judged as being absent, as described above.
Also, because the configuration of the circuit of FIG. 13 judges an input signal as being present if only one pulse is input thereto, even in the case in which no input data but only noise is present within the monitoring interval time, the circuit of FIG. 13 judges that a data signal is present within the time. However, because an optical signal input to an optical receiving device includes a lot of noise components, there is a disadvantage that the circuit of FIG. 13 described above can not be used at all for a signal-off detection circuit of an optical receiving device in an optical transmission system for handling such an optical signal.